How can we assess whether anthropogenic activity has influenced the El Niño-Southern Oscillation (ENSO), an ocean-atmosphere system that influences weather worldwide? A detailed record of changes in the frequency and intensity of ENSO would be helpful, but direct instrumental data extend back only to about 1950, and spatial differences in the expression of the oscillation require documentation over wide geographic areas. In order to construct a history of ENSO long enough to evaluate its decadal variability, proxies such as coral records must be used.

Linsley et al. have measured 101 years of stable oxygen and carbon isotope ratios in coral from Clipperton Atoll, in the eastern Pacific Ocean, in order to examine the past behavior of ENSO. Their finding that oscillation was less intense between 1925 and 1940 provides independent confirmation of similar findings in the central and western Pacific and in the southwest Indian Ocean. They also find a long-term trend in the oxygen isotopes that could be due to a combination of surface ocean warming and decreasing salinity. The decadal variability they observe appears to be related, at least in part, to variation in the Pacific Decadal Oscillation and North Pacific sea surface temperatures, which they suggest is due to changes in surface ocean circulation.—HJS